Astronomy is the science dealing with all the celestial objects in the universe, including the planets and their satellites (e.g. our Earth and the moon), comets and meteors, the stars (including our sun), and interstellar matter, the star systems known as galaxies, and clusters of galaxies.

Ancient peoples watched the stars and memorised their positions relative to each other by visualising constellations - shapes which suggested to them mythical figures. If a new star appeared its position was noted on a star map. The first comet was recorded this way, over 2,000 years ago.

Today, astronomers use telescopes (optical, radio, and others) to study stars, planets, and galaxies. Astronomers are scientists who study all the objects in the universe, such as stars , planets , and Galaxies . They use ground-based telescopes of many kinds, launch space probes that visit the other planets in the solar system, and send satellites into space to study the universe from high above the Earth's atmosphere.

Astronomy analyses the radiation received on Earth or its vicinity, from the constituent parts of the Universe. For most of man's history, the visible part of the electromagnetic spectrum was the sole medium used for astronomical observation, and the human eye was the only receiver. The invention of the optical telescope greatly enhanced our view of the night sky, revealing spectacular details of the Moon and planets, and masses of previously unseen stars and nebulae.

Instruments such as photometers (which measure light intensity) and spectrometers (which split light into its spectrum, or constituent colors), have made the physical study of stars possible, leading to knowledge of their nature, structure, and evolution. These instruments, and photographic plates and electronic imaging devices (e.g. charge-coupled detectors, or CCDs - now used in the consumer market such as in digital cameras) have made possible the field known as astrophysics -  the aplication of physics to astronomy, and have extended the range of the accessible spectrum outside the visible portion.

Astronomers are no longer limited to the visible or near-visible spectrum. Radio telescopes, invented in the 1930s, and the discoveries they made have revolutionised astronomy, and thanks to space technology, we now have access to the whole electromagnetic spectrum, as well as particles from the Sun and other objects. Spacecraft carry telescopes and other astronomical instruments above the Earth's atmosphere, and to other planets in our solar system. This has created whole new branches of astronomy such as X-ray and Gamma-ray astronomy.

    can detect radio objects that are extremely far away and are hard or impossible to see optically. Pulsars, quasars, and radio galaxies were discovered this way. The 'mirror' of a radio telescope is typically a dish of metal or wire mesh. Some radio telescopes are simply rows of wire antennae in a field. Signals from radio telescopes are analysed by computers; and if the data from several telescopes are properly combined, the results can be equivalent to a single telescope much larger than the individual ones involved.

Infrared (IR)

    is the part of the near-optical spectrum we perceive as heat rays, and these are emitted by stellar objects such as the Sun. IR is detected by satellites and ground telescopes, revealing the centers of galaxies, and gas clouds ('nebulae') where stars are being born.

Ultraviolet (UV)

    is at the other end of the optical spectrum from IR, and can be used to learn the composition of stars.

X-rays

    can be received by satellites carrying special detectors. These satellites have observed supernovae, and black holes which emit X-rays as they suck in gases from nearby stars.

Gamma rays

    have very high energy, and come from many objects, e.g. galaxies, and pulsars - the remnants of novae.

Neutrinos

    are elementary particles with no rest mass and no electric charge; they pass through virtually everything, unimpeded. Neutrino detectors placed deep underground can intercept a few of them, and the data used to analyse the Sun and novae.

Visible light